Switch in uhf bandpass

ABSTRACT

By using ICs with combined inputs for the VHF-high bandpass circuit and the UHF bandpass circuit, undesirable resonances occur which are caused by one of the tuned circuits. The problem is solved by providing a switching means for decoupling the UHF bandpass circuit from the combined inputs when a VHF-high band signal is received by the VHF-high bandpass circuit. The switching means comprises a switch coupled between the UHF bandpass circuit and the common inputs.

[0001] The invention relates to a tuner with a UHF bandpass circuit anda VHF bandpass circuit.

[0002] U.S. Pat. No. 6,225,865 discloses a switching circuit forswitching among antennas, which have different directivities. Theswitching circuit provides insulation for an “off” signal in VHF/UHFbands of frequencies without requiring an additional attenuationcircuit. It includes an amplifier for amplifying an input signal and aswitching device for coupling the amplified signal to a common output.The switching circuit also comprises a control circuit, which isresponsive to a first state of a control signal for simultaneouslycausing the switching device to be in a conductive state and enablingthe amplifier to amplify the input signal. The control circuit is alsoresponsive to a second state of the control signal for simultaneouslycausing the switching device to be in a non-conductive state andremoving a DC operating voltage from the amplifier for furtherinsulating the input signal from the output.

[0003] It is, inter alia, an object of the invention to decouple a firstbandpass circuit from common inputs when a second bandpass circuitpresents an output signal on the common inputs.

[0004] The present invention solves the described problem by providing atuner comprising: a first bandpass circuit for receiving a first signalin a first broadcast band and having first output terminals to supply afirst output signal, a second bandpass circuit for receiving a secondsignal in a second broadcast band and having second output terminals tosupply a second output signal, a mixer circuit having common inputs forreceiving both the first output signal and the second output signal, anda switching means for decoupling the first bandpass circuit from atleast one of the common inputs when the second output signal should bepresent on the common inputs.

[0005] According to one embodiment of the invention, the first broadcastband is the UHF band and the second broadcast band is the VHF-H band.The switching means comprise a switch coupled between at least one ofthe first output terminals and the corresponding one of the commoninputs.

[0006] In accordance with another feature of the invention, theswitching means comprises a switch for short-circuiting the first outputsignal and the switch is a diode which is controllable by a biasvoltage.

[0007] The invention may be better understood by referring to theenclosed drawing in which:

[0008]FIG. 1 shows, in a schematic diagram form, an embodiment of a UHFdouble tuned bandpass filter;

[0009]FIG. 2 shows, in a schematic diagram form, an embodiment of a UHFdouble tuned bandpass filter with two switching diodes;

[0010]FIG. 3 shows, in a schematic diagram form, an embodiment of a UHFdouble tuned bandpass filter with two switching diodes and reversebiasing; and

[0011]FIG. 4 shows, in a schematic diagram form, an embodiment of a UHFdouble tuned bandpass filter with a single switching diode.

[0012]FIG. 1 shows an embodiment of a first bandpass circuit and asecond bandpass circuit connected to a mixer circuit having commoninputs for both the first and the second bandpass circuit. The firstbandpass circuit is a typical UHF bandpass filter 1 for receiving asignal in a UHF band and having first output terminals to supply thesignal. The second bandpass circuit is a VHF High bandpass filter 4 forreceiving a signal in a VHF band and having second output terminals tosupply the signal. Both output terminals are connected to a combinedinput 2, 3 of a mixer circuit. The mixer circuit is a Mixer andOscillator Phase Locked Loop (MOPLL/MO) IC 7.

[0013] A signal received by the UHF bandpass filter 1 reaches the IC 7through coupling elements. Coupling elements are a coil 5, which isconnected in series with the capacitor 6 to the input 2 of the IC 7 andcoil 8 and capacitor 9, which is connected in series with the input 3 ofthe IC 7. When the VHF High bandpass filter 4 receives a signal,undesirable resonances occur while tuning in the VHF-High band 4. Theseundesirable resonances are found by components of the UHF bandpasscircuit 1.

[0014] In FIG. 2, a switching means such as an additional switchingdiode 10 is connected in series between the coil 5 and capacitor 6 ofthe UHF double-tuned bandpass filter shown in FIG. 1. A secondadditional switching diode 11 is connected in series between the coil 6and capacitor 9. A control current is fed to the connection of theswitching diode 10 via a resistor 12. A second control current is fed tothe connection of the switching diode 11 via a resistor 13. At the pointof connection of the diode 10 and the capacitor 6, a shunt resistor 14is connected to ground. At the point of connection of the diode 11 andthe capacitor 9, a shunt resistor 15 is also connected to ground. Thediodes 10 and 11 are preferably Schottky barrier or PIN diodes.

[0015] A signal received by the UHF bandpass filter 1 reaches the input2 of the IC 7 through the coil 5, the diode 10 and capacitor 6 and theinput 3 through the coil 8, the diode 11 and capacitor 9. Diodes 10, 11act as a switch by blocking a signal when VHF-High bandpass circuit 4receives a signal. This prevents the undesirable resonances caused bythe UHF bandpass circuit 1 and the signal reaches the inputs 2, 3 of IC7.

[0016]FIG. 3 shows the additional switching diodes 10, 11 with reversebiasing. The diodes are reversed by making use of the IC DC biasing. Thediode 10 is directly connected to the input 2 of the IC 7 without theshunt resistor 14 and capacitor 6 as shown in FIG. 2. The diode 11 isdirectly connected to the input 3 of the IC 7 without the shunt resistor15 and capacitor 9 as shown in FIG. 2.

[0017] A signal received by the UHF bandpass filter 1 reaches the input2 of the IC 7 through the coil 5, the diode 10 and the input 3 throughthe coil 8, and the diode 11. Diodes 10, 11 act as a switch by blockinga signal when VHF-High bandpass circuit 4 receives a signal. This hasthe advantage that the undesirable resonances are prevented, and thecapacitance is futher reduced when the diodes are switched OFF.

[0018] In FIG. 4 the UHF bandpass filter 1 is connected to the inputs 2and 3 through the coupling elements, i.e. coils 5, 8, and capacitors 6,9 as described in FIG. 1. The VHF High bandpass filter 4 is alsoconnected to the combined input 2, 3 of the IC 7 as shown in FIG. 1. Anadditional switching diode 16 is connected to the points of connectionof the UHF bandpass filter 1 and the respective coils 5, 8. A controlcurrent is supplied to the diodes 16 by a resistor 17, which isconnected to the points of connection of the UHF bandpass filter 1 andthe coil 5.

[0019] A signal received by the UHF bandpass filter 1 reaches the IC 7through the coil 5, 8, and capacitor 6, 9. When the VHF-High bandreceives a signal, the UHF bandpass filter 1 is short-cut by theadditional diode 16. Thus, the received signal reaches the inputs 2, 3of the IC 7.

1. A tuner comprising: a first bandpass circuit for receiving a firstsignal in a first broadcast band and having first output terminals tosupply a first output signal, a second bandpass circuit for receiving asecond signal in a second broadcast band and having second outputterminals to supply a second output signal, a mixer circuit havingcommon inputs for receiving both the first output signal and the secondoutput signal, and a switching means for decoupling the first bandpasscircuit from at least one of the common inputs when the second outputsignal should be present on the common inputs.
 2. A tuner as claimed inclaim 1, wherein the first broadcast band is the UHF band, and thesecond broadcast band is the VHF-H band.
 3. A tuner as claimed in claim1, wherein the switching means comprises a switch coupled between atleast one of the first output terminals and the corresponding one of thecommon inputs.
 4. A tuner as claimed in claim 1, wherein the switchingmeans comprises a switch for short-circuiting the first output signal.5. A tuner as claimed in claim 3 or 4, wherein the switch is a diodewhich is controllable by a bias voltage.